The present invention relates to a process for the production of Maddrell's salt by the condensation of sodium dihydrogen phosphate and/or disodium dihydrogen pyrophosphate at 250.degree. to 500.degree. C.
A preferred use of Maddrell's salt is as a cleaning material in toothpastes. Consequently, there is required the smallest possible proportion of water-soluble components, which consist of preponderantly acidic sodium pyrophosphate, sodium trimetaphosphate, and soluble sodium polyphosphate. In the case of the usual production of Maddrell's salt, by thermal condensation of monosodium orthophosphate, the proportion of water-soluble byproducts is dependent upon the reaction conditions, especially the temperature used, and generally is about 5 percent. Accordingly, an object of the present invention is to provide a new and faster process for the formation of Maddrell's salt with a corresponding improvement of the quality thereof.
Because of its importance, the knowledge about Maddrell's salt is relatively great and several processes are known for its production.
According to F. Thilo and R. Ratz (258 Z. Anorg. Allg. Chem. 53 (1949)), Maddrell's salt is produced by heating monosodium dihydrogen phosphate for several days at 350.degree. C. and then washing with water to remove water-soluble by-products which are still present in very considerable quantities.
D. E. Corbridge and F. R. Tromans (30 Analytical Chemistry 1101-1110 (1958)) also describe the formation of Maddrell's salt by heating monosodium orthophosphate to 380.degree. C. for several days and subsequently washing the tempered product with water.
Federal Republic of Germany Patent Specification No. 16 67 569 describes a process for the production of Maddrell's salt from sodium dihydrogen phosphate by heating to 450.degree. C., with the water vapor resulting from the condensation being removed at water vapor partial pressures of from 50 to 450 Torr.
According to Federal Republic of Germany Patent Specification No. 21 61 600, Maddrell's salt may be obtained by the condensation of monosodium orthophosphate at temperatures of from 300.degree. to 380.degree. C. in the presence of water vapor, with the condensation being carried out in a saturated water vapor atmosphere.
It is known from J. R. van Wazer (Phosphorus and its compounds, Vol. I, p. 668) that, in the case of the condensation of monosodium orthophosphate to give Maddrell's salt at 400.degree. C., water vapor arises due to the splitting off of the water of constitution which, under certain circumstances, can have a negative effect on the formation of the Maddrell's salt.
U.S. Pat. No. 2,356,799 discloses a process for the production of Maddrell's salt which has a proportion of water-soluble material of about 4 percent. In a first process step, monosodium orthophosphate is converted into pellets which, by heating to a temperature of from 300.degree. to 460.degree. C., react to give Maddrell's salt. This process is laborious and, because of the additional pelletizing step, is also cost-intensive.
In the case of repeating another known process disclosed in 81 J. Am. Chem. Soc. 79 (1959), it was found that the water-soluble proportion of the end product could not be reduced to below 5 percent, referred to the substance, either by pelletizing or by very fine grinding of the monosodium orthophosphate, or by especially long thermal treatment.
German Democratic Republic Patent Specification No. 137,573, discloses that Maddrell's salt can be produced with a metaphosphate content of less than 2 percent when at least 2 parts of powdered Maddrell's salt are mixed with at most one part of sodium orthophosphate and the mixture is heated, at a rate of more than 50.degree. C. per minute, to 300.degree. to 400.degree. C. and then tempered. In this process, only a one-third yield is obtained and a high control expense is needed.
Furthermore, German Democratic Republic Patent Specification No. 142 959 discloses that Maddrell's salt is formed when monosodium orthophosphate is heated with at least 15% amorphous disodium diphosphate.
Federal Republic of Germany Patent Specification No. 25 15 370, discloses a process for the production of Maddrell's salt by heating monosodium orthophosphate or disodium pyrophosphate at a temperature of from 250.degree. to 450.degree. C., with the reaction rate being increased by adding catalytic amounts of nitrogen-containing bases.
It is therefore possible to start from the process disclosed, for example, in Federal Republic of Germany Patent Specification No. 17 92 703, for the production of chain-shaped, condensed ammonium phosphates, the speed of formation of which can be accelerated by the addition of the base so as to be above the stoichiometrically necessary ratio. The nitrogen-containing base is usually added as a finely-divided powder in the form of the corresponding phosphoric acid salt. In exceptional cases, i.e., only if these salts are not temperature-sensitive, they can also be added to the sodium phosphate before the spraying.
According to all of these processes, a Maddrell's salt is obtained which, besides containing about 5 to 10 percent of by-products which can be washed out with water, also contains, in the interior of the crystals, soluble products which cannot be separated by thorough washing out, and which can only be separated laboriously.
Therefore, the problem exists of finding a new process by which Maddrell's salt can be produced simply and economically, with the salt containing little or no water-soluble components.